Silica-loaded granular rubber and process for producing the same

ABSTRACT

Disclosed are silica-filled rubber granules comprised of a cocoagulation product of rubber and silica, which are extremely less powdery and excellent in handling and kneading, as well as an industrially advantageous process for producing same. The silica-filled rubber granules of the present invention are dried granules of a cocoagulation product of rubber and silica particles, which are characterized in that an average particle diameter (D50) in terms of the sieve analysis is 300˜3000 μm and the weight ratio of the granules within the range of D50±(D50×0.5) is at least 50% by weight. The silica-filled rubber granules are produced by supplying a cake of a cocoagulation product of silica and rubber having a water content of 40˜80% by weight to a drier provided with an indirect-heating type container having stirring wing blades, and then drying the cake while stirring it with the stirring wing blade.

TECHNICAL FIELD

The present invention relates to novel silica-filled rubber granules anda novel process for producing same, and more particularly tosilica-filled rubber granules comprising dry granules of a cocoagulatedsubstance of a rubber and silica particles, which is extremely lesspowdery and good in handling and in kneading property as well as anindustrially beneficial process for producing same.

BACKGROUND ART

From the past, carbon black and silica are widely used as reinforcingfillers for rubber (referred to hereinafter simply as reinforcingmaterial). In general rubber is incorporated with such fillers widely byway of a dry process wherein a kneading apparatus such as a Banburymixer, an open roll, or a kneader is employed.

According to the dry process, however, much more kneading energy andtime are required to obtain a rubbery composition incorporated withfillers. Mentioned as the reason therefor is that a rubber such as astyrene-butadiene copolymer rubber or butadiene rubber is in the form ofa bale so that tremendous shearing power is needed at the initial stageof kneading and a large amount of fillers is to be dispersed into suchrubber that is difficult in kneading.

With a view to reducing kneading energy and time in the dry process, aprocess for obtaining a rubbery composition incorporated with fillershas been investigated wherein a rubber latex and fillers are mixed in anappropriate proportion, and then rubber in the rubber latex iscoagulated with a coagulating agent such as an acid or a salt therebyincorporating the coagulated rubber uniformly with the fillers, or inother words, cocoagulation.

With respect to a process for obtaining a cocoagulation product ofrubber containing silica as filler, there are proposed a method oftreating silica with an alkyltrimethylammonium salt (patent literature1), a method of dispersing silica together with a silylating agent intoa rubber latex (patent literature 2), a method of treating silica withan organosilicon compound (patent literature 3) and a method of treatingsilica with a cationic polymer (patent literature 4).

In these patent literatures, however, there is no disclosure on aconcrete manner from solid-liquid separation to drying of the resultantcocoagulation product and on the form of the resultant silica-filedrubber.

In case a method using a hot blast drier followed by a bale shaping,which is adopted as a general technique for conventional cocoagulationproducts of carbon black, is used for obtaining a dried product of thecocoagulation product of silica and rubber, the silica particles in therubber are strong in their mutual action so that the dried product areobtained as an extremely hard block. A problem arises in such blocksince crushing followed by kneading of the block at the time of shapingis difficult. Looking at the production method, silica is of hydrophilicproperty and gives a high aqueous content in the resultant coagulationproduct so that a longer period of time is needed for drying, thusoffering a problem in industrial operation.

These problems are caused by the fact that silica is extremely higher inhydrophilic property than carbon black.

On the other hand, there are disclosed a process for obtaining a powderysilica-filled rubber according to a method wherein an aqueouscocoagulation product obtained by cocoagulation of silica and rubber issubjected as such or after concentration to spray-drying, oralternatively, the aqueous cocoagulation product is well dried by theaid of a filter press or centrifugal separator and subjected tofluidized drying in a fluidized bed (patent literature 5) and accordingto a method wherein the cocoagulation product after solid-liquidseparation is shaped into the silica-filled rubber in the form ofpellets (patent literature 6).

However, the powder obtained according to the spray-drying has arelatively small average particle diameter of about 100 μm and easilybecomes dusty. Further, the powder attaches to apparatus due to staticelectricity so that a problem arises in any weighing error.

The powders obtained by drying due to the fluidized bed are more or lessadjustable in average particle diameter according to the degree ofpulverization, but the particle size distribution of the resultantpowders, including those attached each other in the fluidized bed andthose more finely pulverized, becomes broader. Likewise the powderobtained by spray-draying, therefore, a problem also arises in thegeneration of dust caused by very fine power and in any weighing errorcaused by attaching of the power to apparatus due to static electricity.

The aforesaid pellets becomes appreciably hard after drying and theirgranular size is large, usually 5˜10 mm so that the effect of reducingshearing power at the initial stage of kneading is small. In casemutually attaching power of the pellets is strong, the pellets tend toinitiate blocking during storage. Looking at the process for production,therefore, a problem arises in necessity of a separate step of makingthe pellets.

Patent literature 1: U.S. Pat. No. 4,482,657

Patent literature 2: JP-A-11-286577

Patent literature 3: JP-A-10-231381

Patent literature 4: JP-A-2003-113250

Patent literature 5: JP-A-2000-351847

Patent literature 6: JP-A-2003-160668

DISCLOSURE OF THE INVENTION

Accordingly, it is an object of the present invention to providesilica-filled rubber granules wherein a cocoagulation product of rubberand silica is in the form of dried granules of extremely less powderynature which are good in handling and kneading and to provide anindustrially advantageous process for producing the silica-filled rubbergranules.

The present inventors have made an extensive study with a view tosolving the aforesaid technical problems and, as a result thereof, ithas now been found that a cake of a cocoagulation product having aspecific moisture content which is obtained by mixing rubber latex withsilica followed by cocoagulating the mixture is dried by the aid of aspecific drier while applying shearing power to the cake, thereby toobtain a granular product almost devoid of finely divided particleswhich has never been realized as a conventional cocoagulation product inthe form of dried solid and that the granular product is therefore lesspowdery nature, less blocking during storage but good in fluidizingproperty and thus excellent in handling so that the product is suitablysubjected to a kneading work by the aid of a conventional kneadingapparatus employed in kneading of rubber. The present invention has beenaccomplished on the basis of the above finding.

In accordance with the present invention, there is providedsilica-filled rubber granules comprising dried granules of acocoagulation product of rubber and silica particles having an averagearticle diameter (D50) in terms of the sieving analysis of 300˜3000 μmand having a weight ratio of the granules within the range ofD50±(D50×0.5 of greater than 50% by weight.

In accordance with the present invention, there is also provided anindustrially very advantageous process for producing silica-filledrubber granules including the above process for producing thesilica-filled rubber granules

In accordance with the present invention, there is further provided aprocess for producing silica-filled rubber granules which comprisesfeeding a cake of a cocoagulation product of silica and rubber having amoisture content of 40˜80% by weight to a drier provided with anindirect-heating vessel fitted with stirring blades, stirring the cakewith the stirring blades under shearing power, and drying the cake.

In the aforesaid process of the present invention, silica-filled rubbergranules of an extremely uniform grain size and in the form ofapproximate spheres can be obtained by carrying out the drying treatmentwhile filtering a liquid containing the cocoagulation product andadjusting the moisture content of the cake to 55˜80% by weight. Suchsilica-filled rubber granules are excellent in design and particularlyexcellent in handling as in fluidity, etc and kneading property at thetime of molding.

By the term “approximate spheres” is meant herein a shape of sphereshaving a sphericity of at least 0.6 wherein the true spherical degree isdetermined by a ratio of the major axis (D_(L)) and the minor axis(D_(S)) of the granules, i.e.( D_(S)/D_(L)).

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is an optical microscopic photograph showing the granularstructure of a silica-filled rubber granules obtained according to theprocess shown in Example 3.

THE BEST MODE FOR CARRYING OUT THE INVENTION

(Rubber)

In the rubber used in the present invention, no limitation exists so faras it can obtain a cocoagulation product with silica. Illustrative ofthe rubber are, in general, diene rubbers such as butadiene rubber,isoprene rubber, butadiene-isoprene copolymeric rubber,styrene-butadiene copolymeric rubber, styrene-butadiene-isoprenecopolymeric rubber, acrylonitrile-butadiene copolymeric rubber, andacrylonitrile-styrene-butadiene copolymeric rubber; and synthetic rubbersuch as chloroprene rubber, butyl rubber and acrylic rubber; and naturalrubber. Besides, denatured rubber wherein functional groups such ashydroxyl groups, carboxyl groups, alkoxysilyl groups, amino groups, andepoxy groups have been introduced into these synthetic rubbers can beused. These rubbers can be used singly or in combination of more thanone.

An oil-extended rubber wherein the rubber is mixed with an extending oilis also used as the aforesaid rubber. Extender oils usually used inrubber industry are used as such extending oils. Examples of theextender oil include petroleum type softening agents of paraffin series,aromatic series and naphthene series, softening agents of plant series,and fatty acids. The softening agents of petroleum series are preferablythose containing less than 3% multi-ring aromatic compounds. Thiscontent is measured according to the IP346 method (the testing method ofTHE INSTITUTE PETROLEUM, U.K.)

The rubber used in the present invention preferably has a Mooneyviscosity (ML1+4, 100° C.) within the range of 100˜200, desirably30˜150.

In case the silica-filled rubber granules are used for tire, the use ofa diene-type rubber such as styrene-butadiene copolymeric rubber andbutadiene rubber is preferable.

(Silica)

No special limitation exists in the present invention with respect tothe silica to be added as filler to the rubber. Mentioned, for example,are precipitated silica prepared by neutralization reaction (theso-called wet process) of an alkali silicate with a mineral acid, drysilica obtained by burning silicon tetrachloride in oxygen-hydrogenflame, and a sol-gel silica obtained by hydrolysis of a silicon alkoxidesuch as tetramethoxysilane or tetraethoxysilane in an aqueous organicsolvent. In case of the precipitated silica, a part of a mineral acid oraluminum sulfate in lieu of the mineral acid is used for theneutralization reaction and the resultant precipitated silica containinga large amount of the metal salt can also be used. These silicamaterials can be used alone or in combination of at least two.

Among these silica materials, the precipitated silica excellent inreinforcing property for rubber and in productivity is preferable in thepresent invention.

As a more detailed description for the aforesaid precipitated silica,the specific surface area (S_(BET)) in terms of the nitrogen adsorptionmethod of the precipitated silica is preferably 70˜300 m²/g, morepreferably 80˜280 m²/g, and more preferably 90˜260 m²/g.

Further, the specific surface area (S_(CTAB)) of the silica measured byadsorption of cetyltrimethylammonium bromide (CTAB) is preferably 60˜300m²/g, more preferably 70˜280 m²/g, and most preferably 80˜260 m²/g.

Furthermore, the oil absorption amount of the silica with butylphthalate (referred to hereinafter simply as “oil absorption amount”) is100˜400 ml/100 g, more preferably 110˜350 ml/100 g, and most preferably120˜300 ml/100 g.

In case a silica having a specific surface area and oil absorptionamount within the above-mentioned ranges is used, the reinforcingproperties such as tensile strength and anti-abrasive property of theresultant silica-filled rubber granules and cross-linked rubber obtainedtherefrom by cross-linking are especially remarkable. Two or more of thesilica having different specific surface area and oil absorption amountwithin the above-mentioned range may be used in combination.

(Cocoagulation Product)

In the present invention, a rubber latex and silica particles in theabove-mentioned proportion are mixed and dispersed thereby coagulatingrubber in the rubber latex and simultaneously incorporating the silicainto the rubber. The degree of coagulation is such that both componentscannot be separated merely by washing with water. Thus, it is presumedthat the rubber is incorporated with the silica.

Taking hardness of the resultant silica-filled rubber granules andphysical properties of the cross-linked rubber into consideration, theproportion of the silica to the rubber in the aforesaid cocoagulationproduct is generally 20˜300 parts by weight, preferably 30˜250 parts byweight, more preferably 40˜200 parts by weight per 100 parts by weightof the rubber.

(Character of the Silica-Filled Rubber Granules)

A character of the silica-filled rubber granules of the presentinvention resides in distribution of the granule size. Namely, thesilica-filled rubber granules of the present invention is characterizedin that the granules are comprised of the aforesaid cocoagulationproduct and its average particle diameter (D50) in terms of the sievinganalysis is 300˜3000 μm, preferably 500˜2000 μm and the weight ratio ofthe granules within the range of 50±(50×0.5) is at least 50% by weight,preferably 80% by weight.

The aforesaid average particle diameter (D50) stands for a value in caseof the accumulated weight percentage remained on the sieve being 50%.

The silica-filled rubber granules comprised of a cocoagulation productof the rubber and the silica particles having such distribution ofgranule size have been proposed for the first time by the presentinvention. In comparison with the conventional cocoagulation product inthe form of pellets, powders or bales, the silica-filled rubber granulesof the present invention are of the following advantages:

-   (1) Advantage Over the Cocoagulation Product in the Form of Pellets:

The silica-filled rubber granules of the present invention are excellentin fluidity so that conveyance becomes easy and energy needed for theinitial stage of kneading can be minimized.

-   (2) Advantage Over the Cocoagulation Product in the Form of Powders:

The use of the silica-filled rubber granules of the present inventioninhibits the generation of dust on kneading and minimizes attachment ofthe dust to the apparatus due to static electricity so that accuracy atthe time of automatic measurement is warranted.

-   (3) Advantage Over the Cocoagulation Product in the Form of Bales:

The use of the silica-filled rubber granules of the present inventionminimizes kneading time and kneading energy so that productivity can bepromoted.

In case an average particle diameter of the cocoagulation product isless than 300 mμ, the generation of dust easily tend to take place sothat a problem arises in handling, especially in generation of dustduring working. On the other hand, if an average particle diameter ofthe cocoagulation product exceeds 3000 μm, there would be anxiety aboutlowering of fluidity and any reduction in minimizing effect of kneadingenergy at the initial stage.

In case the weight ratio of the dried granules within the range of theaforesaid D50±(50×0.5) is less than 50% by weight, fine powders andcoarse granules are increased so that problems of the generation ofdust, any adhesion of powders to the apparatus, etc. will take place.

A preferable embodiment of the silica-filled rubber granules of thepresent invention are those containing not more than 30% by weight offine powders of 200 μm or less, preferably not more than 20% by weightof the fine powders.

(Process for Producing the Silica-Filled Rubber Granules)

No special limitation exists in the process for producing thesilica-filled rubber granules of the present invention, Mentioned as anindustrially advantageous method is a process wherein a cake having amoisture content of 40˜80% by weight obtained by filtering a liquidcontaining the cocoagulation product of silica and rubber is fed to adrier equipped with a vessel fitted with stirring wing blades and anindirect heater thereby drying the cake while stirring it by thestirring wing blades.

As the liquid containing the cocoagulation product of silica and rubberused in the present invention, a liquid obtained by mixing silica with arubber latex and subjecting the mixture to cocoagulation can be usedwithout any limitation. So far as a method for obtaining a cocoagulationproduct wherein silica is incorporated uniformly into the rubber isconcerned, no limitation exists in methods for obtaining cocoagulationproduct of silica and rubber, and any of the known techniques can beapplied. For example, the process disclosed in the aforesaid U.S. Pat.No. 4,482,657, JP-A-11-286577, JP-A-10-231381, or JP-A-2003-113250 canbe used therefor. More precisely, a process wherein silica is treatedwith an alkyltrimethylammonium salt, a silylating agent, anorganosilicon compound or a cationic high molecule to enhance affinityto rubber, and an aqueous suspension of the silica is mixed with therubber to effect cocoagulation of the silica and the rubber ismentioned.

In general, the liquid containing the cocoagulation product of silicaand rubber obtained according to the above process has a solidconcentration of 2˜20% by weight and the particle diameter of thecocoagulation product in the liquid is preferably 50˜5000 μm, preferably80˜2000 μm. A detailed explanation is given hereinafter on the liquidcontaining the cocoagulation product of the silica and the rubber.

(Rubber Latex)

As the rubber latex used in the present invention, the aforesaid rubberlatex can be used without any limitation. A rubber latex stabilized withan anionic emulsifier, a non-ionic emulsifier or a cationic emulsifiermay be used as the rubber latex.

The concentration of rubber in the rubber latex is not especiallydefined and may be defined conveniently according to the purpose and theuse of the rubber. Usually, the concentration is preferably with therange of 5˜80% by weight.

If necessary, the rubber latex may be incorporated with an antioxidant.Illustrative of the antioxidant are, for example, antioxidants of phenoltype such as 2,6-di-tert-butyl-4-methylphenol,octadecyl-3-(3′,3′-di-tert-butyl-4-hydroxylphenol) propionate,styrenated phenol, etc; antioxidants of sulfur type such as2,4-bis(octylthiomethyl)-6-methylphenol, antioxidants of an amine typesuch as N-(1,3-dimethylbutyl)-N′-phenyl-p-phenylenediamine; antioxidantsof quinoline type such as 2,2,4-trimethyl-1,2-dihydroquinoline;antioxidants of hydroquinone type and antioxidants of phosphorus type.

(Aqueous Suspension of Silica)

In the production process of the present invention, the aforesaid silicais used without any special limitation as silica constituting theaqueous silica suspension. The concentration of the silica in theaqueous suspension of silica is adequately 3˜30% by weight, preferably5˜20% by weight.

(Cocoagulation)

In the process of the present invention, any of the known methods isemployed without special limitation a method for mixing the rubber latexwith the aqueous suspension of silica. The use of process disclosed inthe above-mentioned JP-A-2003-113250 is preferable since the silica isincorporated into the rubber at a high probability.

It is recommended to make cocoagulation of the silica with rubber in therubber latex concurrently with or after mixing the rubber latex with theaqueous suspension of silica containing a cationic high molecule or thelike method.

Especially in case the rubber latex stabilized with an anionicemulsifier is used, a part or all of the rubber is coagulated togetherwith silica according to a reaction of the cationic high molecule withthe anionic emulsifier thereby incorporating the silica into the rubber.

In general, cocoagulation of silica and a rubber latex stabilized withthe anionic emulsifier is finished by the action of the cationic highmolecule. For completion of the coagulation of rubber, however, aninorganic acid such as sulfuric acid, phosphoric acid, hydrochloricacid, or the like; a Lewis acid such as aluminum sulfate, or the like;or a salt such as sodium chloride or potassium chloride can be used atneed.

(Adjustment of the Moisture Content of the Cake)

In the process for producing the silica-filled rubber granules of thepresent invention, the liquid containing the cocoagulation product ofthe silica and the rubber obtained according to the aforesaid process isfiltered and, if necessary, dehydrated so that the moisture content ofthe cake becomes 40˜80% by weight, preferably 45˜75% by weight, andespecially 55˜75% by weight. If the moisture content of the cake is lessthan 40% by weight, the cake becomes too hard to crush it into pieces sothat granules having a great granule size and a wide distribution ofgrain size tend to be formed easily. If the moisture content of the cakeexceeds 80% by weight, attachment of the cake to the inside of drier isincreased to reduce the drying efficiency.

No limitation exists in the method for filtration followed by drying,and any of the known apparatus can be used therefor. Applicable to themethod are, for example, a screen, a centrifuge, a decanter, a filterpress, etc. Among these, the filter press is preferable since it isapplicable in case the concentration and particle diameter of thecocoagulation product contained in the liquid are changed and in casethe moisture content of the cake is controlled adequately by varying thecompressed pressure (dehydration).

Conditions in case of using the filter press for the above-mentioneddehydration are not limited, but the liquid containing the cocoagulationproduct is usually fed to a filtering chamber of the filter press underfiltering pressure of 100˜400 KPaG by a pump. No particular limitationexists in the pump for applying the liquid and any of the known pumpscan be used. For example, any of the centrifugal pumps, diaphragm pumps,mohno pumps or the like can be used but the mohno pump is especiallypreferable.

Next, the cake accumulated in the filtering chamber is compressed so asto obtain the cake having a moisture content of 30˜80% by weight. Apressure during the compressing operation is preferably 0˜2000 KPaG,more preferably 300˜1800 KPaQ and especially preferably 600˜1400 KPaG.If the moisture content of the cake is within the above-mentioned range,however, the compressing operation would be unnecessary. Compressed aircan be used up to a pressure of 700 KPaG. However, water pressure shouldbe utilized if the compressed pressure exceeds 700 KPaG.

(Dividing of the Cake)

In the present invention, dividing of the cake prior to feeding it to adrier is preferable for obtaining the granules having a sharp sizedistribution. In particular, diving is effective in case of drying thecake having a moisture content of not more than 75%. Dividing of thecake is not especially limited and a known conventional device can beemployed. For example, any of the apparatus such as a stirring typecrusher, a crusher type dividing machine, a ball-mill type dividingmachine or the like can be used and plural dividing machines may be usedin combination as the case may be. No limitation exists in the method ofdividing (batchwise or continuous), the shape of the stirrer, the shapeof the shaft, the number of the shafts, the length of the shaft, theshape of the crusher portion, and the material of the balls, so far asdividing is possible.

Among the above-mentioned dividing machines, an apparatus provided incombination with a stirring type dividing machine capable of dividingthe cake having a wide range of moisture contents of 40˜80% and atwo-shafts screw type dividing machine is especially preferable.

For example, the cake discharged from a filter press is fed to a hopperequipped with a stirring type dividing machine to effect a primarydividing. In case a two-shafts screw type dividing machine is fitted tothe lower part of the hopper, a secondary dividing is effected in situand the divided cake is thrown into a drier. In case of dividing thecake having a moisture content of 40˜55%, the granule diameter at thefinal stage of dividing is preferably 5 mm or less, more preferably 3 mmor less and most preferably 1 mm or less so that the granules afterdividing may be made more uniform in granule diameter. If the granulediameter at the final stage exceeds 5 mm, the granules tend to becomenon-uniform in granule diameter. The two-shafts screw type dividingmachines enable to make self-cleaning of the screw grooves so that it iseffective for the cake within the range of moisture content exhibitingattaching property.

(Drying)

In the present invention, no limitation exists in the drier for dryingthe cake or a divided material thereof and any of the known conventionaldriers can be used so far as the drier is capable of applying shearingforce to the cake and is comprised of an indirect-heating containerhaving stirring wing blades. The mode of drying may be either batchwiseor continuous. The shape of either vertical type or horizontal type canobtain the granules, but in a drying process considering productivity,the continuous mode and the horizontal shape are preferable. In case ofthe production amount is small, one drier is sufficient enough fordrying the case. In case of the amount is large, a serial arrangement ofat least two driers is generally better in production efficiency.

In this case, the drying operation for forming the granules ispreferably carried out in the indirect-heating container having stirringwing blades. No limitation exists in the drier to be established afterthe above drying operation. More concretely, after-drying operation ismost preferably carried out by arranging at least two indirect-heatingcontainers having stirring wing blades of the same type or a differenttype in shearing force (preferably smaller). As another embodiment, theindirect heating container having a strength to a certain degree havebeen obtained by the front stage drying with the indirect-heatingcontainer having stirring wing blades capable of applying shearingforce, the drying operation is carried out while inhibiting generationof fine dust,

In general, the drying time in the indirect-heating container havingstirring wing blades capable of affording shearing force which isadvantageous for the formation of the granules is 2˜3 minutes,especially 3˜20 minutes in terms of residential time, although the valueis somewhat changeable according to the moisture content.

An explanation on the concrete embodiment of the indirect-heatingcontainer having stirring wing blades is as follows: In general, aheating medium is supplied to a jacket provided with the outside of thecontainer, but besides the jacket, the heating medium is preferablysupplied to the stirring shaft and stirring wing blades so that theheat-conductive area becomes larger. Used as the heating medium are anoil, steam, and warmed water. Feeding of the cake is carried out in anyof the modes such as a batchwise mode, an intermittent mode, and acontinuous mode. So far as the known conventional feeder is used, thereis no particular limitation. As described above, however, the use of thetwo-shafts screw-type dividing machine can omit the use of any feeder.

Concerning the stirring, no limitation exists and a known conventionalstirring wing blade can be used so far as its shape is a wing bladecapable of applying shearing force to the fed cake or a divided productthereof and capable of stirring it for drying. For example, any of thewing blades such as inclined rod wing blades, inclined plate wingblades, helical ribbon wing blades, anchor wing blades, disk wingblades, and scraping wing blades can be used. No special limitationexists in the number of stirring shafts, but the number is preferablyone to three with a view to simplifying the structure of the apparatus.The circumferential velocity of the stirring wing blades is preferablyfaster within the possible range of the apparatus for refreshing thecake on the heat-conductive surface in the container to promoteheat-conductive efficiency. More concretely, the velocity is 0.3˜10 m/s,preferably 2˜10 m/s, and most preferably 4˜10 m/s.

In the drier comprised of the indirect-heating container, the clearance(t) between the stirring wing blades and the wall of the container isadjusted to 2˜50 mm, preferably 5˜35 mm so that a more proper shearingforce can preferably be applied to the cake of the cocoagulation productin the drier at the circumferential velocity of the stirring wing bladesthereby obtaining granules having more uniform distribution of granulesize.

In the aforesaid drier, the portion where the clearance (t) between thewall of the container and the stirring wing blades is within theabove-mentioned range is preferably in the portion where the granularshape is formed. All of the portions in the drier is not necessary toadjust the clearance so as to have the value within the above range.More concretely, the above clearance is provided in the portion aroundthe feeding hopper of the drier where the granular shape is formed.

The temperature of the cake or a divided product thereof in the courseof drying is preferably not higher than 130° C. by adjusting the jackettemperature or the feeding amount of the cake or a divided productthereof in order to inhibit degradation of rubber. In accordance withthe structure of the drier, it is also possible to control thetemperature of the composition more stably by dividing the jacket intoplural portions and controlling them with a heating medium having thesame temperature or a different temperature.

In the present invention, a gas is preferably passed through the drierlest the water vapor evaporated in the drier should be condensed again,in order to dry the cake or a divided product thereof efficiently.Mentioned as the gas to be passed through the drier is air, nitrogen orother inert gas. No limitation exists in the temperature of the passinggas so far as the temperature does not permit the generation of dew. Nolimitation also exists in the flowing direction of the gas. In case ofthe continuous drying method, however, it is preferable to flow the gasfrom the exit of the composition to the entrance.

No special limitation exists in the internal pressure of theindirect-heating container during the drying and normal pressure orreduced pressure can be applied. However, drying under reduced pressureis preferable since the composition can be controlled at a lowertemperature. Even in case the drying operation is carried out underreduced pressure, passing of the above mentioned gas through thecontainer is effective.

Completion of the drying of the silica-filled rubber granules of thepresent invention is preferably at the stage where the moisture contentof the composition becomes 5% by weight or less, especially 3% by weightor less. A composition having a moisture content exceeding 5% by weightnot only fails to exhibit good physical properties but also brings aboutbad influence on use.

Fluidizing of the composition by stirring or discharging of thecomposition taking advantage of removal is preferable as a method fordischarging the dried composition from the drier. In case of thecontinuous drying, however, the flowing gas must be sealed. Thus, it isgeneral to use a method wherein the discharge portion of the compositionis provided with a rotary valve or the like to seal the flowing gas.

Since the evaporated water vapor and the flowing gas from the drier maybe entrained with the granules having a small particle diameter, adust-collecting device is preferably fitted to the discharge portion ofthe drier. No special limitation exists in the dust-collecting device sothat any known conventional device can be used. The evaporated watervapor and the flowing gas are allowed to pass through a low temperaturecondenser to recover the evaporated water alone. If impurities such asstyrene and the like are contained in the recovered water, anappropriate treatment is carried out for discharged water. In this case,the drying process of the present invention minimized the amount ofdischarge gas as compared with the spray drying and the fluidized beddrying so that the labor for treating such discharge gas can remarkablybe reduced.

(Other Additives)

The silica-filled rubber granules obtained according to the process ofthe present invention is incorporated with known conventional additivesusually incorporated into rubber thereby to produce a rubber compositionwhich is cross-linked, if necessary, for practical use.

Illustrative of such additive are, for example, additional other rubber;additional other silica; a filler such as carbon black, carbon-silicadual phase filler which is carbon black carried on the surface thereofwith silica, talc, clay, calcium carbonate, aluminum hydroxide, andstarch; an oil-extending oil; a silane-coupling agent; a cross-linkingagent; a cross-linking accelerator; a cross-linking activating agent; anantioxidants; an activating agent; a process oil; a plasticizer; alubricant; and a scorching-preventing agent. Further, the silica-filledrubber granules can be incorporated with the above as diluting rubber.

Illustrative of the other additional rubber are, for example, apolyether rubber such as epichlorohydrin rubber, a fluorine rubber, asilicone rubber, an ethylene-propylene-diene rubber, and an urethanerubber. These are selectively used according to the requiredcharacteristics. These other additional rubbers can be used singly or incombination of at least two.

Illustrative of the silane-coupling agent are, for example,vinyltriethoxysilane, β-(3,4-epoxycyclohexyl)ethyltrimethoxysilane,N-(β-aminoethyl)-γminopropyltrimethoxysilane,3-octathio-1-propyltriethoxysilane, a sulfide such asbis[3-(triethoxysilyl)propyl]tetrasulfide,bis[3-(triethoxysilyl)propyl]disulfide;γ-trimethoxysilylpropyldimethylthiocarbamyl tetrasulfide as disclosed inJP-A-6-248116, and γ-trimethoxysilylpropylbenzothiazyl tetrasulfide.Since the scorching at the time of kneading is avoided, thesesilane-coupling agents preferably contain not more than 4 sulfur atomsper molecule. These silane-coupling agents can be used singly or incombination with at least two.

The proportion of the silane-coupling agent is preferably 0.1˜20 partsby weight, more preferably 0.5˜15 parts by weight, and most preferably1˜10 parts by weight per 100 parts of silica.

Illustrative of the carbon black are furnace black, acetylene black,thermal black, channel black and graphite. Among these, the furnaceblack is preferable. More concretely, mentioned are SAF, ISAF, ISAF-HS,ISAF-LS, IISAF-HS, HAF, HAF-HS, HAF-LS, and FEF.

The above-mentioned carbon black can be used singly or in combination ofat least two. The proportion of the carbon black is usually not morethan 150 parts by weight and a total of the carbon black and the silicais preferably 20˜200 parts by weight per 100 parts by weight of therubber.

The BET specific surface area of the carbon black is not specificallylimited but is preferably 30˜200 m²/g, more preferably 50˜150 m²/g, andmost preferably 70˜140 m²/g.

The oil-absorption amount of the carbon black is preferably 30˜300ml/100 g, more preferably 50˜200 ml/100 g, and most preferably 80˜160ml/100 g.

Illustrative of the cross-linking agent are sulfur such as powderysulfur, precipitated sulfur, colloidal sulfur, insoluble sulfur, orhighly dispersed sulfur; a halogenated sulfur such as sulfurmonochloride or sulfur dichloride; an organic peroxide such as dicumylperoxide or di-tert-butyl peroxide; a quinone dioxime such as p-quinonedioxime or p,p-dibenzoylquinone dioxime; an organic polyamine compoundsuch as triethylene tetramine, hexamethylenediamine carbamate, or4,4′-methylene-bis-o-chloroaniline; and an alkylphenol resin havingmethylol groups. Among these, sulfur is preferable and powdery sulfur isespecially preferable. These cross-linking agents are used singly or incombination of at least two.

The proportion of the cross-linking agent is preferably 0.1˜15 parts byweight, more preferably 0.3˜10 parts by weight, and especiallypreferably 0.5˜5 parts by weight per 100 parts by weight of the rubbercomponent. In case the cross-linking agent is within the aforesaidrange, the rubber granules are excellent in fuel-saving property andreinforcing property.

Illustrative of the cross-linking accelerator are a sulfenamide typecross-linking accelerator such asN-cyclohexyl-2-benzothiazylsulfenamide,N-tert-butyl-2-benzothaizolsulfenamide,N-oxyethylene-2-benzothiazolsulfenamide, orN,N-diisoropyl-2-benzothiazolsulfenamide; a guanidine type cross-linkingaccelerator such as diphenylguanidine, di-o-tolylbiguanidine, oro-tolylguanidine; a thiourea type cross-linking accelerator such asdiethylthiourea; a thiazole type cross-linking accelerator such as2-mercaptobenzothiazole, dibenzothiazyl sulfide, or2-mercaptobenzothiazole zinc salt; a thiuram type cross-linkingaccelerator such as tetramethylthiuram monosulfide ortetramethylthiouram disulfide; a diethyldithio carbamate typecross-linking accelerator such as sodium dimethyldithiocarbamate or zincdiethyldithiocabamate; and a xanthate type cross-linking acceleratorsuch as zinc isopropylxanthate or zinc butylxanthate.

These cross-linking accelerators are used singly or in combination of atleast two and those containing the sulfenamide type cross-linkingaccelerator are especially preferable.

The proportion of the cross-linking accelerator is preferably 0.1˜15parts by weight, more preferably 0.3˜10 parts by weight and especiallypreferably 0.5˜5 parts by weight per 100 parts by weight of the rubbercomponent.

No special limitation exists in the cross-linking activating agent, buta higher fatty acid such as stearic acid or a zinc oxide can be used.Preferable is the use of zinc oxide having high surface activity and aparticle diameter of not more than 5 mμ. Mentioned is an active zincwhite having a particle diameter of 0.05˜0.2 mμ or a zinc white having aparticle diameter of 0.3˜1 mμ. A zinc oxide having been treated on thesurface thereof with a dispersing agent of an amine type can be used.

These cross-linking accelerators can be used singly or in combination ofat least two.

The proportion of the cross-linking activating agent is appropriatelychosen according to the sort of the cross-linking activating agent. Theproportion of the higher fatty acid is preferably 0.05˜15 parts byweight, more preferably 0.1˜10 parts by weight, and especiallypreferably 0.5˜5 parts by weight per 100 parts by weight of the rubbercomponent. The proportion of the zinc oxide is preferably 0.05˜10 partsby weight, more preferably 0.1˜5 parts by weight, and especiallypreferably 0.5˜3 parts by weight per 100 parts by weight of the rubbercomponent.

Illustrative of the other additives are an activating agent such asdiethylene glycol, polyethylene glycol, and a silicone oil; and a wax.

(Cross-Linked Rubber)

Cross-linked rubber obtained by cross-linking the silica-filled rubbergranules of the present invention can be obtained by kneading thesilica-filled rubber granules of the present invention in accordancewith a usual method with the respective components followed bycross-linking. For example, the silica-filled rubber granules and otherrubbers are kneaded with the additives and fillers except thecross-linking agent and the cross-linking accelerator, and thereaftermixing the kneaded mixture with the cross-linking agent and thecrosslinking accelerator to form a cross-linkable composition. Akneading time of the silica-filled rubber granules and other rubberstogether with the additives and filler except the cross-linking agentand the cross-linking accelerator is preferably from 30 seconds to 30minutes while a kneading temperature is within the range of preferably80˜200° C., more preferably 100˜190° C., and especially preferably140˜180° C. Next, the resultant kneading mixture is cooled down topreferably 100° C. or less, more preferably 80° C. or less, andthereafter kneaded with the cross-linking agent and the cross-linkingaccelerator.

In the present invention, no special limitation exists in the method ofkneading, and suitably chosen according to the nature of thecross-linked rubber and magnitude thereof, etc. The cross-linkablerubber composition is preferably charged into a metal mold and heated toeffect molding and cross-linking concurrently or alternatively apreviously shaped cross-linkable composition is heated to effectcross-linking. The cross-linking temperature is preferably 120˜200° C.,more preferably 140˜180° C. while the cross-linking time is usuallyabout 1˜120 minutes.

EFFECT OF THE INVENTION

The silica-filled rubber granules of the present invention is suppressedin the amount of existing fine powders so that the granules areextremely less powdery and excellent in handling and in kneading. Theresultant cross-linked rubber composition which has been obtained bycross-linking the silica-filled rubber granules, wherein the silica andthe rubber are homogeneously mixed is excellent in reinforcingproperties such as tensile strength and anti-abrasive property.

Accordingly, the silica-filled rubber granules of the present inventioncan be utilized for the use taking advantage of the characteristicproperties of the granules, for example, tire parts such as tread,under-tread, carcass, side-wall, and bead; rubbery articles such ashose, window frames, belt, shoe tread, vibration-preventing rubber, autoparts, and vibration-suppressing rubber; and resin-reinforced rubberyparts such as impact-resisting polystyrene and ABS resin. Among these,the rubber granules of the present invention are suitable for use intire parts and especially suitable as tire tread for fuel-saving tires.

EXAMPLES

The present invention will now be illustrated more in detail by way ofExamples and Comparative Examples, but it is to be construed that thepresent invention is not limited by these Examples. By the way, variousphysical properties given in these Examples and Comparative Exampleswere measured according to the following methods:

-   -   (1) An average particle diameter of silica in an aqueous        suspension and an average particle diameter of a cocoagulation        product of silica and rubber:        -   Using a light-scattering diffraction type particle size            distribution measuring apparatus (Model LS-230 manufactured            by Beckman Coulter Inc.), a volume median particle size was            measured and the value was adopted as an average particle            diameter.    -   (2) Specific surface area:    -   Measurement of the specific surface area (S_(BET)) according to        the nitrogen absorption method:        -   A silica wet cake was placed in a drier (120° C.) for            drying, and thereafter using Model ASAP 2010 manufactured by            Micrometritix, the nitrogen adsorption amount was measured            and the value according to the single point analysis at            relative pressure 0.2.    -   Measurement of the specific surface area (S_(CTAB)) by        adsorption of cetyltrimethyl ammonium bromide (CTAB):        -   A silica wet cake was placed in a drier (120° C.) for            drying, and thereafter the measurement was carried out in            accordance with the method described in ASTM D3765-92. As            the method described in ASTM D3765-92 was a method for            measurement of S_(CTAB) of carbon black, however, this            testing method was somewhat improved. More precisely,            without using ITRB (83.0 m²/g) which is a standard sample of            carbon black, a standard liquid of CTAB was prepared            separately. An assay of Aerosol OT solution was carried out            by using the liquid, and a specific surface area of the            silica was calculated from the adsorption amount of CTAB,            assuming that the adsorption cross-sectional area per mole            of CTAB to the surface area of the silica is 35 square Å.            This is due to the reason that as the surface condition is            different between carbon black and silica, the adsorption            amount of CTAB is considered to be different between them            even in case of the same specific surface area.    -   (3) Oil-absorption amount:        -   This value was obtained in accordance with JIS K6220.    -   (3) Amount of styrene unit in the copolymer:        -   This value was obtained in accordance with JIS K6383 (the            refractive index method).    -   (4) The ratio of silica content:        -   Using a thermal analysis apparatus (model: TG/DTA320            manufactured by Seiko Electronic Industry), a residual ratio            and a weight loss up to 150° C. after pyrolysis of a dried            sample in the air were measured and the ratio of silica            content was calculated according to the following equation.            In Examples, the calculated value was converted into a ratio            of an amount (parts by weight) to 100 parts by weight of the            rubber. The measuring conditions were as follows: a            temperature elevation velocity in the air of 20° C./min., a            reached temperature of 600° C., and a retention time at            600° C. of 20 minutes.        -   The ratio of silica content (%)=a residual ratio/[100−(a            weight loss up to 150° C.)]×100.    -   (5) Average particle diameter (D50) of silica-filled rubber,        distribution index of article diameter (P) and weight ratio (W)        of fine powder of not more than 200 μm:        -   Using a vibrating sieve-machine (manufactured by Tanaka            Chemical Machinery Co.), six mesh sieves of a proper size            (JIS Z8801) were equipped to the machine and 20 g of a            sample was charged into the machine and vibrated for 5            minutes, and thereafter the weight of the sample on each            sieve was measured. The particle diameter (D50) of the            sample in case of the weight percentage of the sample            retained on the sieves becoming 50% in term of weight, the            ratio (W) % of the fine powder of 200 μm or less, and the            weight ratio (P) of the dried granules within the range of            D50±(D50×0.5) were then calculated. As the value of P            becomes larger, the particle diameter is more uniform.    -   (7) Sphericity:        -   20 Typical particles were arbitrarily selected from a            photographic image obtained by an optical microscope, the            major axis (D_(L)) and the minor axis (D_(S)) of the            particle images were measured by a scale. The sphericity is            an average value of the ratio (D_(S)/D_(L)).    -   (8) 300% Modulus, tensile strength:        -   This value was measured by means of a tensile test in            accordance with JIS K6301.    -   (9) Abrasion index:        -   Using an Acron type abrasion tester, the abrasion index was            calculated from loss in weight after 1000 times preliminary            abrasion and after 1000 times abrasion. As the abrasion            index becomes larger, the abrasion property becomes better.            (10)    -   Workability:    -   Degree of the generation of dust:        -   At the time of kneading the silica-filled rubber granules,            especially at the time of feeding them to a Banbury mixer,            degree of the generation of dust was evaluated according to            the following classes:    -   ⊚ No dust    -   ◯ Slightly generated    -   Δ Generated somewhat largly    -   × Generated seriously    -   Fluidity:        -   Using a Powder Tester (Hosokawa Micron Co., LTD.), the            degree of compression, the angle of repose, the angle of            spatula and uniformity were measured, and the Carr's            fluidity index was calculated from these values [A Japanese            book entitled “Funtai Bussei Zusetsu” published on May 1 of            Showa 50 (May 1, 1975) by Funtai kogaku Kenkyusho (Research            Association of Powder Technology) and Nihon Funtai Kogyo            Kyokai (Society of Powder Tecnology), pages 146˜151]. As the            value of the fluidity index becomes larger, the fluidity            becomes better.    -   Time necessary to reach the maximum torque:        -   The silica-filed rubber was charged into a Banbury mixer and            kneaded. A period of time from the initiation of kneading to            the kneading torque becoming the maximum was measured. As            the period of time becomes shorter, the kneaded mass is more            easily packed into the mixer so that the silica becomes            homogeneous in the rubber.    -   (11) Kneading energy:        -   Using a Banbury mixer (Laboplast Mill Model 100C, Mixer type            B-250), A total energy needed for kneading was measured.

Example for Producing SBR Latex

In a pressure-resistant reactor equipped with a stirrer were placed 200parts of deionized water, 1.5 parts of a rosin acid soap, 2.1 parts offatty acid soap, 0.20 part of tert-dodecylmercaptan, and, as monomers,72 parts of 1,3-butadiene and 28 parts of styrene. The temperature ofthe reactor was set at 10° C., and 0.03 part of diisopropylbenzenehydroperoxide as a polymerization initiator, 0.04 part of sodiumformaldehyde sulfoxide, 0.01 part of sodium ethylenediaminetetraacetate,and 0.03 part of ferric sulfate were added to the reactor to initiatethe polymerization. At the time the polymerization conversion ratereached to 45%, 0.05 part of tert-dodecylmercaptan was added to continuethe reaction. At the time the polymerization conversion rate reached to70%, 0.05 part of diethylhydroxylamine was added to terminate thereaction.

After removing unreacted monomers by steam distillation, a 30% aqueousemulsified solution of 0.8 parts ofoctadecyl-3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate and 0.12 partsof 2,4-bis(n-octylthiomethyl)-6-methylphenol was added as antioxidantsper 100 parts of the polymer thereby to obtain a polymer latex (referredto hereinafter as SBR latex) having a solid concentration of 24%.

Next, a part of the latex was taken out and the latex was coagulated at50° C. by addition of sodium chloride while adjusting the pH of thelatex to 3˜5 to obtain a polymer in the form of crumb. This crumb wasdried in a hot blast drier at 80° C. to obtain a solid rubber (referredto hereinafter as solid SBR). The resultant rubber had a styreneproportion of 23.6% by weight and a Mooney viscosity of 52. The solidSBR thus obtained was used in the following Example 2.

Example 1 of Producing Silica

In a 1 m³ stainless steel reactor equipped with a thermo-regulator wasplaced 230 liters of an aqueous solution of sodium silicate (SiO₂concentration: 10 g/liter and the molar ratio: SiO₂/Na₂O=3.41) and thetemperature of the solution was raised to 85° C. Next, 73 liters of 22%sulfuric acid and 440 liters of an aqueous solution of sodium silicate(SiO₂ concentration: 90 g/liter and the molar ratio: SiO₂/Na₂O=3.41)were concurrently added in 120 minutes. After aging for 10 minutes, 16liters of 22% sulfuric acid was added in 15 minutes. The above reactionwas carried out by stirring the reaction liquid at all times whilemaintaining the reaction temperature at 85° C. to obtain a silica slurryhaving a pH of 3.2 finally. The silica slurry was washed with water andfiltered by a filter press to obtain a silica wet cake (A) having asilica solid of 23%.

A part of the resultant silica wet cake (A) was dried to obtain silicapowders (a) having a BET specific surface area (S_(BET)) of 201 m²/g, aCTAB specific surface area (S_(CTAB)) of 190 m²/g, and an oil absorptionamount of 210 ml/100 g.

The silica wet cake (A) obtained according to the above process and purewater were divided and mixed by a homogenizer so that a silica solidconcentration of the aqueous suspension might be 15%. Next, a cationicpolymer (polydiallylmethylammonium chloride having a weight averagemolecular weight: 20,000) was admixed in an amount such that 3 parts byweight was used for 100 parts of the silica solid concentration, therebyobtaining a cationic polymer-containing silica aqueous suspension (I). Asilica particle diameter of the aqueous suspension (I) was 15 μm.

Example 2 of Producing Silica

In a 1 m³ stainless steel reactor equipped with a thermo-regulator wasplaced 200 liters of an aqueous solution of sodium silicate (SiO₂concentration: 10 g/liter and the molar ratio: SiO₂/Na₂O=3.41) and thetemperature of the solution was raised to 95° C. Next, 77 liters of 22%sulfuric acid and 445 liters of an aqueous solution of sodium silicate(SiO₂ concentration: 90 g/liter and the molar ratio: SiO₂/Na₂O=3.41)were concurrently added in 140 minutes. After aging for 10 minutes, 16liters of 22% sulfuric acid was added in 15 minutes. The above reactionwas carried out by stirring the reaction liquid at all times whilemaintaining the reaction temperature at 95° C. to obtain a silica slurryhaving a pH of 3.2 finally. The silica slurry was washed with water andfiltered by a filter press to obtain a silica wet cake (B) having asilica solid of 25%.

A part of the resultant silica wet cake (B) was dried to obtain silicapowders (b) having a BET specific surface area (S_(BET)) of 201 m²/g, aCTAB specific surface area (S_(CTAB)) of 110 m²/g, and an oil absorptionamount of 170 ml/100 g.

The silica wet cake (B) obtained according to the above process and purewater were divided and mixed by a homogenizer so that a silica solidconcentration of the aqueous suspension might be 15%. Next, a cationicpolymer (polydiallylmethylammonium chloride having a weight averagemolecular weight: 20,000) was admixed in an amount such that 3 parts byweight was used for 100 parts of the silica solid concentration, therebyobtaining a cationic polymer-containing aqueous silica suspension (I). Asilica particle diameter of the aqueous suspension (II) was 15 μm.

Example 1 for Producing a Liquid Containing a Cocoagulation Product ofSilica and Rubber

26.7 Kilograms of the aqueous silica suspension (I) was diluted with 90kg of pure water and warmed up to 50° C. To the diluted aqueous silicasuspension was then added 33.9 kg of the SBR latex under agitation toobtain a liquid (A) containing a cocoagulation product of silica andSBR. An average particle diameter of the cocoagulation product in theliquid (A) containing the cocoagulation product of silica and SBR was100 μm. A part of the liquid (A) was filtered and dried to obtain thecocoagulation product having a silica content of 49 parts by weight.

Example 2 for Producing a Liquid Containing a Cocoagulation Product ofSilica and Rubber

26.7 Kilograms of the aqueous silica suspension (II) was diluted with 90kg of pure water and warmed up to 50° C. To the diluted aqueous silicasuspension was then added 33.9 kg of the SBR latex under agitation toobtain a liquid (B) containing a cocoagulation product of silica andSBR. An average particle diameter of the cocoagulation product in theliquid (B) containing the cocoagulation product of silica and SBR was800 μm. A part of the liquid (B) was filtered and dried to obtain thecocoagulation product having a silica content of 49 parts by weight.

Example 3 for Producing a Liquid Containing a Cocoagulation Product ofSilica and Rubber

53.4 Kilograms of the aqueous silica suspension (1) was diluted with 90Kg of pure water and warmed up to 50° C. To the diluted aqueous silicasuspension was then added 33.9 kg of the SBR latex under agitation toobtain a liquid (C) containing a cocoagulation product of silica andSBR. An average particle diameter of the cocoagulation product in theliquid (C) containing the cocoagulation product of silica and SBR was 80μm. A part of the liquid (A) was filtered and dried to obtain thecocoagulation product having a silica content of 98 parts by weight.

Example 1

The liquid (A) containing a cocoagulation product of silica and rubberwas supplied to a filter press by the aid of a diaphragm pump andfiltration was carried out under filtration pressure of 400 kPaG. Next,compression was carried out by using compressed air of 700 kPaG As aresult, a water content of the cake was 51% by weight.

The cake was charged into a hopper equipped with a stirring typedividing machine to effect a primary dividing operation and a secondarydividing operation was then carried out by using a two-shafts screw typedividing machine equipped to the lower part of the hopper, while feedingthe divided cake to a drier. At the time of the secondary dividingoperation, the divided cake had predominantly a particle diameter of 1mm or less.

The drier used was a lateral type internally stirring drier (A) as shownin Table 1, and the drying operation was carried out continuously. Theperformance of the lateral type internally stirring drier (A) and thedrying condition are shown in Tables 1 and 2, respectively.

The temperature just after drying of the resultant silica-filled rubbergranules was 114° C. and the water content thereof was 2.0% by weight.Table 4 shows D50, P, true spherical ratio, W and fluidity of thesilica-filled rubber granules.

The silica-filled rubber granules thus obtained was incorporated with asilane coupling agent (KBE-846 manufactured by Shin-etsu Kagaku Kogyo),paraffin wax, stearic acid, zinc white, and an antioxidant (NOCRAC 6Cmanufactured by Ohuchi Sinko Kagaku Kogyo-sha), and kneaded for 2minutes by the aid of a Banbury mixer (Laboplast Mill Model 100C Mixertype B-250 manufactured by Toyo Seiki).

Table 4 shows a result of workability at the time of kneading. Thetemperature on completion of the kneading was 150° C. Next, vulcanizingaccelerator (NOCCELLER CZ manufactured by Ohuchi Sinko Kagaku Kogyo-sha)and sulfur were added and the whole was kneaded at 70° C. for oneminutes by the aid of a Banbury mixer. A test piece was manufactured bypress-vulcanizing at 160° C. for 15 minutes and measured for variousphysical properties.

By the way, the time for reaching the maximum torque, kneading energy,and measured values of physical properties were shown in term of indexin case of Comparative Example 1 being 100. A result obtained is shownin Table 4.

Example 2

The liquid (C) containing a cocoagulation product of silica and rubberwas supplied to a filter press by the aid of a diaphragm pump andfiltration was carried out under filtration pressure of 400 kPaG. Next,compression was carried out by using compressed air of 700 kPaG. As aresult, a water content of the cake was 60% by weight.

The dividing and drying operations were carried out in the same manneras described in Example 1. The resultant divided cake had predominantlya particle diameter of 1 mm or less. The temperature just after dryingof the resultant silica-filled rubber granules was 105° C. and the watercontent thereof was 2.0% by weight. Table 4 shows D50, P, sphericity, Wand fluidity of the silica-filled rubber granules.

The resultant silica-filled rubber granules was incorporated withvarious additives so as to have the proportion as shown in Table 3, andkneaded and vulcanized as in Example 1 to prepare a test piece. Thephysical properties of the test piece were measured, and the result ofworkability on kneading is shown in Table 4.

By the way, the time for reaching the maximum torque, kneading energy,and measured values of physical properties were shown in term of indexin case of Comparative Example 1 being 100. A result obtained is shownin Table 4.

Example 3

The liquid (A) containing a cocoagulation product of silica and rubberwas supplied to a filter press by the aid of a diaphragm pump andfiltration was carried out under filtration pressure of 100 kPaG. Next,compression was carried out by using compressed air of 250 kPaG. As aresult, a water content of the cake was 72.5% by weight.

The cake was easily capable of being divided and supplied continuouslyto the drier only by the aid of a two-shafts screw type dividingmachine.

The drier comprised of a lateral type internally stirring drier (B)arranged serially after a laterally type internally stirring drier (A)and the drying operation was carried out continuously. The performance othe lateral type internally stirring drier (B) and the drying conditionare shown in Tables 1 and 2, respectively.

The temperature just after drying of the resulting silica-filled rubbergranules was 118° C. and the moisture content thereof was 0.9% byweight. Table 4 shows D50, P. sphericity, W and fluidity of thesilica-filled rubber granules.

The resultant silica-filled rubber granules was incorporated withvarious additives so as to have the proportion as shown in Table 3, andkneaded and vulcanized as in Example 1 to prepare a test piece. Thephysical properties of the test piece were measured, and the result ofworkability on kneading is shown in Table 4.

By the way, the time for reaching the maximum torque, kneading energy,and measured values of physical properties were shown in term of indexin case of Comparative Example 1 being 100. A result obtained is shownin Table 4.

Example 4

Using the cake employed in Example 3 having a water content of 72.5% byweight and a two-shafts screw type dividing machine alone, the cake wascontinuously supplied to the drier.

The drier used was a lateral type internally stirring drier (A) as shownin Table 1, and the drying operation was carried out continuously in thesame manner as described in Example 1 except that the drying conditionwas as shown in Table 2.

The temperature just after drying of the resultant silica-filled rubbergranules was 106° C. and the water content thereof was 1.5% by weight.Table 4 shows D50, P, sphericity, W and fluidity of the silica-filledrubber granules.

The resultant silica-filled rubber granules was incorporated withvarious additives so as to have the proportion as shown in Table 3, andkneaded and vulcanized as in Example 1 to prepare a test piece. Thephysical properties of the test piece were measured, and the result ofworkability on kneading is shown in Table 4.

By the way, the time for reaching the maximum torque, kneading energy,and measured values of physical properties were shown in term of indexin case of Comparative Example 1 being 100. A result obtained is shownin Table 4.

Example 5

The liquid (B) containing a cocoagulation product of silica and rubberwas supplied to a filter press by the aid of a diaphragm pump andfiltration was carried out under filtration pressure of 400 kPaG. Next,compression was carried out by using compressed air of 700 kPaG As aresult, a water content of the cake was 45% by weight. The dividing anddrying operation was carried out in the same manner as in Example 1. Thedivided cake had predominantly a particle diameter of 1 mm or less.

The temperature just after drying of the resultant silica-filled rubbergranules was 120° C. and the water content thereof was 1.0% by weight.Table 4 shows D50, P, sphericity, W and fluidity of the silica-filledrubber granules.

The resultant silica-filled rubber granules was incorporated withvarious additives so as to have the proportion as shown in Table 3, andkneaded and vulcanized as in Example 1 to prepare a test piece. Thephysical properties of the test piece were measured, and the result ofworkability on kneading is shown in Table 4.

By the way, the time for reaching the maximum torque, kneading energy,and measured values of physical properties were shown in term of indexin case of Comparative Example 1 being 100. A result of the measurementis shown in Table 4.

Comparative Example 1

The cake obtained in Example 1 having a water content of 51% by weightwas dried at 110° C. in a box type hot-blast circulating drier to obtaina lumpy silica-filled rubber. A water content of the resultantsilica-filled rubber was 0.9%, and W was 2% by weight.

After crushing the resultant lumpy mass of silica-filled rubber so thatthe diameter thereof might become about 5 cm, the silica-filled rubberwas incorporated with various additives so as to have the proportion asshown in Table 3, the kneaded and vulcanized as in Example 1 to preparea test piece. The physical properties of the test piece were measured,and the result of workability on kneading is shown in Table 4.

By the way, time for reaching the maximum torque, kneading energy, andmeasured values of physical properties were shown in term of index incase of Comparative Example 1 being 100.

Comparative Example 2 (Fluidized Bed)

The cake obtained in Example 1 having a water content of 51% by weightwas charged into a hopper equipped with a stirring type dividing machineto perform the primary dividing operation and the secondary dividingoperation was then carried out by using the two-shafts screw typedividing machine equipped to the lower part of the hopper, while feedingthe divided cake continuously to a drier. The divided cake at the timeof the secondary dividing operation had predominantly a particlediameter of 1 mm or less.

A continuous type fluidized bed drier was used as the drier, and acontinuous drying operation was carried out by setting the amount ofsupplying the divided cake so as to enable the drying under suchcondition that the inlet gas temperature was 150° C., the outlet gastemperature was 60° C., and the residential time was 30 minutes. Theresultant granular silica-filled rubber just after drying had atemperature of 102° C., and a water content of 2.0% by weight. In thefluidized bed, there was seen such divided cake mutually bound again.Table 4 shows D50, P, sphericity, W and fluidity of the silica-filledrubber powder (A).

The silica-filled rubber granules thus obtained was incorporated with asilane coupling agent (KBE-846 manufactured by Shin-etsu Kagaku Kogyo),paraffin wax, stearic acid, zinc white, and an antioxidant (NOCRAC 6Cmanufactured by Ohuchi Sinko Kagaku Kogyo-sha), and kneaded for 2minutes by the aid of a Banbury mixer (Laboplast Mill Model 100C Mixertype B-250 manufactured by Toyo Seiki).

Table 3 shows presence or absence of the generation of powder dust andthe result of workability. The temperature of the silica-filled rubbergranules just after kneading was 150° C. Next, vulcanizing accelerator(NOCCELER CZ manufactured by Ohuchi Sinko Kagaku Kogyo-sha) and sulfurwere added and the whole was kneaded at 70° C. for one minutes by theaid of a Banbury mixer. A test piece was manufactured bypress-vulcanizing at 160° C. for 15 minutes and measured for variousphysical properties.

By the way, the time for reaching the maximum torque, kneading energy,and measured values of physical properties were shown in term of indexin case of Comparative Example 1 being 100. A result obtained is shownin Table 5.

Comparative Example 3 (Fluidized Bed)

The cake used in Example 3 having a water content of 72.5% by weight wasused and a dividing operation was carried out only by the aid of thetwo-shafts screw type dividing machine. As in the case of ComparativeExample 2, a fluidized bed drying was carried out in trial. As the watercontent of the cake was too excessive, however, the cake was bound tothe bottom plate of the fluidized bed so that the drying operation couldnot well be carried out.

INDUSTRIAL APPLICABILITY

The novel silica-filled rubber obtained according to the presentinvention has such a character that it is extremely less powdery and ishardly charged electrostatically. In addition, it is excellent inhandling and kneading and so has a great possibility of effectivelyutilizing as a reinforcing filler for rubber.

TABLE 1 Unit Drier A Drier B Full length mm 3000 1600 Internal diameterof drum mm 150 400 Number of stirring shafts number 1 1 Shape ofstirring wing — Plate inclined wing Disk wing blades Clearance betweenwall and mm 10 4 wing blades Supply of heating medium — Jacket Jacket,shaft and blades Sort of heating medium — Steam Steam

TABLE 2 Examples 1, 2, 5 Example 3 Example 4 Drier Unit Drier A Drier ADrier B Drier A Residential time Minutes 4 4 20 6 of materials to bedried Circumferential m/s 4 4 0.53 3 velocity of stirring wing bladesTemperature of ° C. 160 160 130 160 jacket Dew-preventing — Air Air AirNitrogen gas Temperature of ° C. 160 160 130 160 dew-preventing gasOperation kPaG Normal Normal Normal Normal pressure pressure pressurepressure pressure

TABLE 3 Examples Comparative Comparative Unit 1, 3, 4 and 5 Example 2Example 1 Example 2 Silica-filled rubber granules phr 150 100 — — SolidSBR phr — 50 — — Silica-filled rubber lump phr — — 150 — Silica-filledrubber powder phr — — — 150 Silane coupling agent phr 4 4 4 4 Paraffinwax phr 1 1 1 1 Stearic acid phr 2 2 2 2 Zinc white phr 4 4 4 4Antioxidant phr 1 1 1 1 Vulcanizing accelerator phr 1.5 1.5 1.5 1.5Sulfur phr 2 2 2 2

TABLE 4 Example 1 Example 2 Example 3 Example 4 Example 5 D50 (μm) 800700 1600 1400 800 P (% by weight) 55 74 85 84 53 Sphericity 0.74 0.750.84 0.85 0.68 W (% by weight) 10 7 3 2 10 Workability Generation ofdust ⊚ ⊚ ⊚ ⊚ ⊚ Fluidity 96 97 98 99 96 Time necessary to 80 82 80 79 80reach the maximum torque Kneading energy 85 88 86 89 82 Physical 300%modulus 100 98 101 101 105 properties Tensile strength 98 97 97 98 95 ofrubber Abrasion index 102 97 100 101 95

TABLE 5 Comparative Example 1 Comparative Example 2 D50 (μm) — 180 P (%by weight) — 28 Sphericity — 0.61 W (% by weight) 2 70 WorkabilityGeneration of ⊚ Δ dust Fluidity — 78 Time necessary 100 90 to reach themaximum torque Kneading energy 100 90 Physical 300% Modulus 100 101properties of Tensile strength 100 96 rubber Abrasion index 100 99

1. Silica-filled rubber granules wherein the granules are comprised ofdried granules of a cocoagulation product of rubber and silica having anaverage particle diameter (D50) of 300-3000 μm and a weight ratio of thegranules within the range of D50±(D50×0.5) is at least 50% by weight,and wherein the granules have a sphericity of 0.68-0.85 determined by aratio of the major axis (D_(L)) and the minor axis (D_(S)) of thegranules (D_(L)/D_(S)).
 2. Silica-filled rubber granules according toclaim 1, wherein a weight ratio of the granules within the range ofD50±(D50×0.5) is at least 80% by weight.
 3. Cross-linked rubber obtainedby cross-linking the silica-filled rubber granules according to claim 1or 2.